Eiger BioPharmaceuticals (EIGRQ) 10-Q 9 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 05, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	EIGR
Entity Registrant Name	Eiger BioPharmaceuticals, Inc.
Entity Central Index Key	1,305,253
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		19,095,526

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 16,927	$ 32,035
Debt securities, available-for-sale	48,013	9,744
Prepaid expenses and other current assets	1,864	712
Total current assets	66,804	42,491
Property and equipment, net	185	79
Other assets	389	312
Total assets	67,378	42,882
Current liabilities:
Accounts payable	6,517	3,183
Accrued liabilities	1,459	2,084
Current portion of long term debt	6,565	2,002
Total current liabilities	14,541	7,269
Long term debt, net	18,856	13,091
Other long term liabilities	214
Total liabilities	33,611	20,360
Stockholders’ equity:
Common stock	14	11
Additional paid-in capital	188,455	141,320
Accumulated other comprehensive loss	(6)	(3)
Accumulated deficit	(154,696)	(118,806)
Total stockholders’ equity	33,767	22,522
Total liabilities and stockholders’ equity	$ 67,378	$ 42,882

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 13,196	$ 6,145	$ 25,080	$ 21,740
General and administrative	3,643	2,727	9,874	9,195
Total operating expenses	16,839	8,872	34,954	30,935
Loss from operations	(16,839)	(8,872)	(34,954)	(30,935)
Interest expense	(681)	(388)	(1,574)	(1,129)
Interest income	371	98	654	321
Other income (expense), net	5	(8)	(16)	188
Net loss	$ (17,144)	$ (9,170)	$ (35,890)	$ (31,555)
Net loss per common share, basic and diluted	$ (1.20)	$ (1.10)	$ (2.92)	$ (3.77)
Weighted-average common shares outstanding, basic and diluted	14,255,843	8,372,934	12,290,500	8,366,880

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (17,144)	$ (9,170)	$ (35,890)	$ (31,555)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale debt securities	8	4	(3)	13
Comprehensive loss	$ (17,136)	$ (9,166)	$ (35,893)	$ (31,542)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flow (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (35,890)	$ (31,555)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	34	30
Amortization of debt securities discounts	(257)	(63)
Non-cash interest expense	393	266
Common stock issued under Product Development Agreement	361
Stock-based compensation	3,496	3,223
Gain on intellectual property sale		(200)
Change in operating assets and liabilities:
Prepaid expenses and other current assets	(1,152)	(202)
Other non-current assets	(77)	77
Accounts payable	3,334	1,528
Accrued and other liabilities	(625)	(1,083)
Other long term liabilities	214
Net cash used in operating activities	(30,169)	(27,979)
Investing activities
Purchase of debt securities available-for-sale	(52,265)	(22,533)
Proceeds from maturities of debt securities available-for-sale	14,250	35,525
Proceeds from intellectual property sale		200
Purchase of property and equipment	(140)	(44)
Net cash (used in) provided by investing activities	(38,155)	13,148
Financing activities
Proceeds from issuance of common stock upon public offering, net of issuance cost	42,890
Proceeds from borrowings in connection with term loan, net of issuance cost	9,935
Proceeds from issuance of common stock upon stock option exercises	295	11
Proceeds from issuance of common stock upon ESPP purchase	96	142
Net cash provided by financing activities	53,216	153
Net decrease in cash and cash equivalents	(15,108)	(14,678)
Cash and cash equivalents at beginning of period	32,035	27,756
Cash and cash equivalents at end of period	$ 16,927	$ 13,078

Description of Business

Description of Business	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of Business Eiger BioPharmaceuticals, Inc. (the “Company”) is a clinical-stage biopharmaceutical company committed to bringing to market novel products for the treatment of rare diseases. The Company innovates by developing well characterized drugs acting on newly identified or novel targets in rare diseases. The Company’s mission is to systematically reduce the time and cost of the drug development process to more rapidly deliver important medicines to patients with rare diseases. Lonafarnib is the Company’s lead compound advancing into 1) Phase 3 in a single, pivotal trial to treat Hepatitis Delta Virus (“HDV”) infection by the end of the year, and 2) NDA for the treatment of Hutchinson-Gilford Progeria Syndrome (HGPS or Progeria) in 2019. The Company’s principal operations are based in Palo Alto, California, and it operates in one segment. Liquidity As of September 30, 2018, the Company had $16.9 million of cash and cash equivalents, $48.0 million of debt securities available-for-sale, an accumulated deficit of $154.7 million and negative cash flows from operating activities. The Company expects to continue to incur losses for the next several years. In October 2018, the Company completed an underwritten public offering of 4,830,918 shares of common stock, including 630,120 shares of common stock purchased to the underwriter’s option to purchase additional shares, at an offering price of $10.35 per share. The Company received net proceeds of approximately $48.0 million, after deducting underwriting discounts and commissions. See Note 11. Management believes that the currently available resources will be sufficient to fund its operations for at least the next 12 months following the issuance date of these unaudited condensed consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The unaudited condensed consolidated financial statements include the accounts of Eiger BioPharmaceuticals, Inc. and its wholly owned subsidiaries, EBPI Merger Inc., EB Pharma LLC and Eiger BioPharmaceuticals Europe Limited, and have been prepared in accordance with accounting principles generally accepted in the United States of America, (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s financial information. These interim results are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any other future year. The balance sheet as of December 31, 2017, has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission on March 9, 2018. Use of Estimates The preparation of unaudited condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates its estimates, including those related to clinical trial accrued liabilities, stock-based compensation and income taxes. The Company bases its estimates on historical experience and on various other market-specific and relevant assumptions that the Company believes to be reasonable under the circumstances. Actual results could differ from those estimates. Debt Securities Short-term debt securities consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than 365 days from the date of acquisition. All short-term debt securities are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale debt securities are excluded from earnings and are reported as a component of accumulated other comprehensive loss. The cost of available-for-sale debt securities sold is based on the specific-identification method . Accrued Research and Development Costs The Company accrues for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced and includes these costs in accrued liabilities in the unaudited condensed consolidated balance sheets and within research and development expense in the unaudited condensed consolidated statements of operations. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers. The Company makes judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. Net Loss per Share Basic net loss per share of common stock is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Since the Company was in a loss position for all periods presented, diluted net loss per share is the same as basic net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive. The following table sets forth the outstanding potentially dilutive securities which have been excluded in the calculation of diluted net loss per share because including such securities would be anti-dilutive (in common stock equivalent shares): Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Options to purchase common stock 2,079,497 1,530,394 2,079,497 1,530,394 Warrants to purchase common stock 10,180 10,180 10,180 10,180 Total 2,089,677 1,540,574 2,089,677 1,540,574 Recent Accounting Pronouncements In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), Targeted Improvements to Leases (Topic 842) Codification Improvements to Leases (Topic 842) The Company is currently in the process of evaluating the impact that the standard will have on its consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) In August 2016, the FASB issued ASU No. 2016-15, "Statement of Cash Flows (Topic 230) Classification of Certain Cash Receipts and Cash Payments. The Company has adopted this guidance during the quarter ended March 31, 2018. The adoption of this guidance did not have a significant impact on the statement of cash flows when adopted. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718) Revenue from Contracts with Customers In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) The Company is currently in the process of evaluating the impact the standard will have on its consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Fair value accounting is applied for all financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis (at least annually). At September 30, 2018 and December 31, 2017, the carrying amount of prepaid expenses, accounts payable and accrued liabilities approximated their estimate fair value due to their relatively short maturities. Management believes the terms of long-term debt reflect current market conditions for an instrument with similar terms and maturity, therefore the carrying value of the Company’s debt approximated its fair value. Assets and liabilities recorded at fair value on a recurring basis in the unaudited condensed consolidated balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1 : Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 : Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 : Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The Company’s money market funds are classified as Level 1 because they are valued using quoted market prices. The Company’s debt securities consist of available-for-sale securities and are classified as Level 2 because their value is based on valuations using significant inputs derived from or corroborated by observable market data. There were no assets or liabilities classified as Level 3 as of September 30, 2018 and December 31, 2017. There were no transfers between Level 1, Level 2 or Level 3 of the fair value hierarchy during the periods presented. The following tables present the fair value hierarchy for assets and liabilities measured at fair value (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 6,681 $ — $ — $ 6,681 Corporate debt securities — 21,722 — 21,722 Commercial paper — 28,537 — 28,537 Total $ 6,681 $ 50,259 $ — $ 56,940 December 31, 2017 Level 1 Level 2 Level 3 Total Financial assets: Money market funds $ 19,612 $ — $ — $ 19,612 Corporate debt securities — 6,501 — 6,501 Commercial paper — 3,243 — 3,243 Total $ 19,612 $ 9,744 $ — $ 29,356 There were no financial liabilities as of September 30, 2018 and December 31, 2017. The following tables summarize the estimated value of the Company’s cash equivalents and debt securities and the gross unrealized holding gains and losses (in thousands): September 30, 2018 Amortized cost Unrealized gain Unrealized loss Estimated Fair Value Cash equivalents: Money market funds $ 6,681 $ — $ — $ 6,681 Corporate debt securities — — — — Commercial paper 2,246 — — 2,246 Total cash equivalents $ 8,927 $ — $ — $ 8,927 Debt securities: Corporate debt securities $ 21,726 $ 1 $ (5 ) $ 21,722 Commercial paper 26,293 1 (3 ) 26,291 Total debt securities $ 48,019 $ 2 $ (8 ) $ 48,013 December 31, 2017 Amortized cost Unrealized gain Unrealized loss Estimated Fair Value Cash equivalents: Money market funds $ 19,612 $ — $ — $ 19,612 Total cash equivalents $ 19,612 $ — $ — $ 19,612 Debt securities: Corporate debt securities $ 6,503 $ — $ (2 ) $ 6,501 Commercial paper 3,244 — (1 ) 3,243 Total debt securities $ 9,747 $ — $ (3 ) $ 9,744 As of December 31, 2017, the contractual maturity of the available-for-sale debt securities is less than one year. The Company periodically reviews the available-for-sale investments for impairment loss. The Company considers factors such as the duration, severity and the reason for the decline in value, the potential recovery period and its intent to sell. For debt securities, it also considers whether (i) it is more likely ot that the Company will be required to sell the debt securities before recovery of their amortized cost basis, and (ii) the amortized cost basis cannot be recovered as a result of credit losses. During the three and nine months ended September 30, 2018, the Company did not recognize any impairment losses.

Accrued Liabilities

Accrued Liabilities	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Accrued Liabilities	4. Accrued Liabilities Accrued liabilities consist of the following (in thousands): September 30, December 31, 2018 2017 Compensation and related benefits $ 1,049 $ 1,262 Contract research costs 196 634 Consulting costs 184 87 Franchise tax 30 56 Contract manufacturing costs — 4 Other — 41 Total accrued liabilities $ 1,459 $ 2,084

License, Collaboration and Prod

License, Collaboration and Product Development Agreements	9 Months Ended
	Sep. 30, 2018
Collaboration And License Agreements [Abstract]
License, Collaboration and Product Development Agreements	5. License, Collaboration and Product Development Agreements Product Development Agreement On August 11, 2018, the Company entered into a Product Development Agreement and a First Project Agreement (the “Product Agreements”), pursuant to which the Company will receive development program support services for its hepatitis Delta virus (“HDV”) program. The services are to be provided from July 1, 2018 through the completion of the Phase 3 Clinical Study Reports and the subsequent NDA filing. As consideration, the Company has committed to pay fees of approximately $10.0 million in cash and stock over four years, including approximately $0.8 million for expert consultant fees and pass through costs to vendors, plus certain incentive-based regulatory milestone fees up to $1.0 million. As part of the aggregate payment, the Company issued 115,526 shares of common stock subject to quarterly vesting requirements related to successful performance of the services and achievement of budget timeline set forth in the Product Agreements. Product Agreements Merck License Agreement On May 15, 2018, the Company entered into an amendment to the license agreement with Merck Sharp & Dohme Corp. (“Merck”) dated September 2, 2010, as amended, which provides for expansion of the existing exclusively licensed field of use under the license agreement with Merck to include all uses of lonafarnib related to the treatment of Hutchinson-Gilford Progeria Syndrome (“HGPS” or “Progeria”) in humans at no cost to the Company. The Company has the sole responsibility and the continuing obligation for the manufacture and supply of lonafarnib to The Progeria Research Foundation (“PRF”). Merck will not receive milestone payments in relation to lonafarnib for the treatment of progeria and progeroid laminopathies or any royalty payments for sales of a specified quantity of lonafarnib to treat the currently estimated progeria and progeroid laminopathies patient population worldwide. Progeria Research Foundation (PRF) Collaboration Agreement On May 15, 2018, the Company entered into a Collaboration and Supply Agreement (the “PRF Collaboration Agreement”) with PRF. Under the PRF Collaboration Agreement, the parties agreed to collaborate with respect to the development and pursuit of regulatory approval of lonafarnib for the treatment of progeria and progeroid laminopathies in humans. PRF granted the Company a non-exclusive, world-wide, royalty-free, sub-licensable license pertaining to all intellectual property and data controlled by PRF to prepare and file any new drug application (“NDA”) for a product containing lonafarnib for progeria and progeroid laminopathies. The Company is obligated to: (i) exclusively supply lonafarnib to PRF for use in clinical trials and non-clinical research in progeria and progeroid laminopathies at the Company’s expense, (ii) prepare and be the sponsor of any NDA submission for lonafarnib for the treatment of progeria and progeroid laminopathies to the FDA, (iii) use commercially reasonable efforts to file a NDA for progeria and progeroid laminopathies by a specified date, (iv) submit a rare pediatric disease designation and a request for expedited approval in connection with a NDA filing, (v) establish a patient support program in progeria and progeroid laminopathies, and (vi) use commercially reasonable efforts to develop a pediatric formulation of lonafarnib for use in progeria and progeroid laminopathies. Under the PRF Collaboration Agreement, the Company is solely responsible for any additional studies necessary for obtaining an NDA for progeria and progeroid laminopathies and is also responsible for any additional costs for such studies up to $2.0 million. The PRF Collaboration Agreement continues for an initial term of ten years and automatically renews for subsequent renewal terms of two years each unless either party terminates earlier. Clinigen Master Service Agreement On April 26, 2018, the Company entered into a master service agreement with Clinigen Healthcare Ltd. (“Clinigen”) in anticipation of its obligations under the PRF Collaboration Agreement to establish an Expanded Access Program for children with progeria and progeroid laminopathies. On May 23, 2018, the Company entered into the first statement of work (“SOW”) under the agreement. Pursuant to the SOW, Clinigen became an authorized non-exclusive worldwide distributor of lonafarnib, the unlicensed pharmaceutical product (the “Product”). The Company is responsible for supply of the Product to Clinigen, and Clinigen is responsible for providing the Product to patients as part of the patient support program. Clinigen is also obligated to set up, manage and close-out the patient support program. The agreement will continue on a country-by-country basis until the Product is commercially available in that country.

Debt

Debt	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt	6 . Debt In December 2016, the Company entered into an aggregate $25.0 million loan with Oxford Finance LLC (the “Oxford Loan”) and borrowed $15.0 million (“Tranche A”). The loan matures on July 1, 2021. In May 2018, the Company entered into an amendment to the Oxford Loan (the “Amendment”) and borrowed $5.0 million (“Amended Tranche B”). On August 3, 2018, the Company The Oxford Loan bears interest at a floating rate per annum equal to the greater of either the 30-day U.S. Dollar LIBOR reported in the Wall Street Journal plus 6.41% or 6.95%. Commencing on the first day of the month following the funding of Tranche A, the Company is required to repay the Tranche A in 18 monthly interest only payments, and starting on August 1, 2018, 36 equal monthly payments of principal and interest. Upon the receipt of Amended Tranche B, the interest only period for borrowed funds was extended by six months until February 1, 2019, followed by 30 equal monthly payments of principal plus accrued interest. at the time of final payment of Amended Tranche B, the Company is required to pay an additional exit fee of $0.1 million direct deduction from the carrying amount of the related debt liability The loan is secured by the perfected first priority liens on the Company's assets, including a commitment by the Company to not allow any liens to be placed upon the Company’s intellectual property. The Oxford Loan includes customary events of default, including failure to pay amounts due, breaches of covenants and warranties, material adverse effect events, certain cross defaults and judgments, and insolvency. If the Company is unable to comply with these covenants or if the Company defaults on any portion of the outstanding borrowings, the lenders can also impose a 5.0% penalty and restrict access to additional borrowings under the loan and security agreement. The Company is permitted to make voluntary prepayments of the Oxford Loan with a prepayment fee, calculated as of the loan origination date, equal to (i) 3.0% of the loan prepaid during the first 12 months, (ii) 2.0% of the loan prepaid in months 13-24 and (iii) 1.0% of the loan prepaid thereafter. The Company is required to make mandatory prepayments of the outstanding loan upon the acceleration by lender following the occurrence of an event of default, along with a payment of the final payment, the prepayment fee and any other obligations that are due and payable at the time of prepayment. The Company accounts for the amortization of the debt discount utilizing the effective interest method. The Company recorded interest expense of $0.7 million and $1.6 million for the three and nine months ended September 30, 2018. Long-term debt and unamortized discount balances are as follows (in thousands): September 30, December 31, 2018 2017 Face value of long term debt $ 25,000 $ 15,000 Exit fee 1,960 1,125 Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees (1,539 ) (1,032 ) Total long term debt 25,421 15,093 Less: current portion of long term debt (6,565 ) (2,002 ) Long term debt, net $ 18,856 $ 13,091

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7 . Stock-Based Compensation The following table summarizes stock option activity under the Company’s stock-based compensation plan during the nine months ended September 30, 2018 (in thousands, except option and share data): Shares Available for Grant Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (in Years) Aggregate Intrinsic Value Outstanding as of December 31, 2017 799,375 1,467,051 $ 12.70 8.41 $ 4,511 Additional options authorized 526,330 Granted (867,500 ) 867,500 $ 10.34 Exercised (39,780 ) $ 7.45 Canceled and forfeited 215,274 (215,274 ) $ 13.36 Outstanding as of September 30, 2018 673,479 2,079,497 $ 11.75 8.33 $ 4,243 Vested and exercisable as of September 30, 2018 842,968 $ 12.62 7.66 $ 2,080 During the three and nine months ended September 30, 2018, the Company granted employees stock options for zero and 799,500 shares, respectively. The weighted-average grant date fair value of these options was $7.50 for the nine months ended September 30, 2018. During the three and nine months ended September 30, 2017, the Company granted employees stock options for 56,000 and 579,700 shares, respectively. The weighted-average grant date fair value of these options was $7.45 and $7.66 for the three and nine months ended September 30, 2017, respectively. The Company records stock-based compensation of stock options granted to employees by estimating the fair value of stock-based awards using the Black-Scholes option pricing model and amortizes the fair value of the stock-based awards granted over the applicable vesting period of the awards on a straight-line basis. The fair value of employee stock options was estimated using the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) — 5.77-6.08 5.27-6.08 5.27-6.08 Volatility — 79.0% 84.0%-84.5% 79.0%-80.0% Risk free interest rate — 1.93%-2.01% 2.35%-2.68% 1.63%-2.23% Dividend yield — — — — Stock-Based Compensation Expense Total stock-based compensation expense recognized for options granted was as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 344 $ 273 $ 1,138 $ 771 General and administrative 828 680 2,358 2,452 Total $ 1,172 $ 953 $ 3,496 $ 3,223 As of September 30, 2018, the total unrecognized compensation expense related to unvested options was $9.7 million, which the Company expects to recognize over an estimated weighted average period of 2.8 years.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8 . Income Taxes The tax expense for the three and nine months ended September 30, 2018 was zero due to the Company’s loss position and full valuation allowance. This is consistent with the zero-tax expense for the three and nine months ended September 30, 2017.

Legal Matters

Legal Matters	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Legal Matters	9 . Legal Matters In July 2015, following Celladon’s announcements of the negative CUPID 2 data and the suspension of further research and development activities and the subsequent declines of the price of its common stock, three putative class actions were filed in the U.S. District Court for the Southern District of California against Celladon and certain of its current and former officers. The complaints generally alleged that the defendants violated Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), by making materially false and misleading statements regarding the clinical trial program for MYDICAR, thereby artificially inflating the price of Celladon’s common stock. The complaints sought unspecified monetary damages and other relief, including attorneys’ fees. On December 9, 2015, the district court consolidated the three On October 7, 2016, the district court granted defendants’ motion to dismiss the consolidated amended complaint and granted leave to amend within 60 days from the date of the district court’s order. The lead plaintiff subsequently filed a notice of intent not to amend the consolidated amended complaint and instead indicated that it intended to appeal the district court’s decision. On December 9, 2016, the district court closed the case. On December 28, 2016, the lead plaintiff filed a notice to the United States Court of Appeals for the Ninth Circuit appealing the district court’s order dismissing the consolidated amended complaint. On May 5, 2017, the lead plaintiff and appellant filed his opening appellate brief. On July 5, 2017, defendants filed their answering appellate brief response. The Plaintiff subsequently filed their response to the Company’s July 5, 2017 filing on August 19, 2017. Oral arguments were heard on August 28, 2018 before the Ninth Circuit Court of Appeals, and on September 17, 2018, the court ruled that the lower court ruling was re-affirmed. The plaintiff had two weeks to file a petition for a rehearing and such filing was not made within the allowed time, effectively terminating the shareholder lawsuit. It is possible that additional suits will be filed, or allegations made by stockholders, with respect to these same or other matters and also naming the Company and/or Celladon’s former officers and directors as defendants. The Company believes that it has meritorious defenses and intends to defend these lawsuits vigorously. Due to the early stage of these proceedings, the Company is not able to predict or reasonably estimate the ultimate outcome or possible losses relating to these claims.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	10 . Commitments and Contingencies Lease Agreement In October 2017, the Company entered into a non-cancelable facility lease agreement for 8,029 square feet of office space located at 2155 Park Blvd. in Palo Alto, California 94306. The lease commenced on March 1, 2018 and expires in February 2023. The lease has one three-year renewal option prior to expiration and includes rent escalation clauses through the lease term. In October 2017, the Company provided a security deposit of $0.3 million. The future minimum rent payable under the new lease agreement is approximately $0.6 million per year. There were no other changes in commitments and contingencies during the three and nine months ended September 30, 2018.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events In October 2018, the Company completed an underwritten public offering of 4,830,918 shares of common stock, including 630,120 shares of common stock purchased to the underwriter’s option to purchase additional shares, at an offering price of $10.35 per share. The Company received net proceeds of approximately $48.0 million, after deducting underwriting discounts and commissions.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The unaudited condensed consolidated financial statements include the accounts of Eiger BioPharmaceuticals, Inc. and its wholly owned subsidiaries, EBPI Merger Inc., EB Pharma LLC and Eiger BioPharmaceuticals Europe Limited, and have been prepared in accordance with accounting principles generally accepted in the United States of America, (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s financial information. These interim results are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any other future year. The balance sheet as of December 31, 2017, has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission on March 9, 2018.
Use of Estimates	Use of Estimates The preparation of unaudited condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates its estimates, including those related to clinical trial accrued liabilities, stock-based compensation and income taxes. The Company bases its estimates on historical experience and on various other market-specific and relevant assumptions that the Company believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Debt Securities	Debt Securities Short-term debt securities consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than 365 days from the date of acquisition. All short-term debt securities are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale debt securities are excluded from earnings and are reported as a component of accumulated other comprehensive loss. The cost of available-for-sale debt securities sold is based on the specific-identification method .
Accrued Research and Development Costs	Accrued Research and Development Costs The Company accrues for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced and includes these costs in accrued liabilities in the unaudited condensed consolidated balance sheets and within research and development expense in the unaudited condensed consolidated statements of operations. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers. The Company makes judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities.
Net Loss per Share	Net Loss per Share Basic net loss per share of common stock is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Since the Company was in a loss position for all periods presented, diluted net loss per share is the same as basic net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive. The following table sets forth the outstanding potentially dilutive securities which have been excluded in the calculation of diluted net loss per share because including such securities would be anti-dilutive (in common stock equivalent shares): Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Options to purchase common stock 2,079,497 1,530,394 2,079,497 1,530,394 Warrants to purchase common stock 10,180 10,180 10,180 10,180 Total 2,089,677 1,540,574 2,089,677 1,540,574
Recent Accounting Pronouncements	Recent Accounting Pronouncements In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), Targeted Improvements to Leases (Topic 842) Codification Improvements to Leases (Topic 842) The Company is currently in the process of evaluating the impact that the standard will have on its consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) In August 2016, the FASB issued ASU No. 2016-15, "Statement of Cash Flows (Topic 230) Classification of Certain Cash Receipts and Cash Payments. The Company has adopted this guidance during the quarter ended March 31, 2018. The adoption of this guidance did not have a significant impact on the statement of cash flows when adopted. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718) Revenue from Contracts with Customers In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) The Company is currently in the process of evaluating the impact the standard will have on its consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Schedule of Antidilutive Securities Excluded from Computation of Earnings per Share	The following table sets forth the outstanding potentially dilutive securities which have been excluded in the calculation of diluted net loss per share because including such securities would be anti-dilutive (in common stock equivalent shares): Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Options to purchase common stock 2,079,497 1,530,394 2,079,497 1,530,394 Warrants to purchase common stock 10,180 10,180 10,180 10,180 Total 2,089,677 1,540,574 2,089,677 1,540,574

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value	The following tables present the fair value hierarchy for assets and liabilities measured at fair value (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 6,681 $ — $ — $ 6,681 Corporate debt securities — 21,722 — 21,722 Commercial paper — 28,537 — 28,537 Total $ 6,681 $ 50,259 $ — $ 56,940 December 31, 2017 Level 1 Level 2 Level 3 Total Financial assets: Money market funds $ 19,612 $ — $ — $ 19,612 Corporate debt securities — 6,501 — 6,501 Commercial paper — 3,243 — 3,243 Total $ 19,612 $ 9,744 $ — $ 29,356
Summary of Estimated Value of Cash Equivalents and Debt Securities and Gross Unrealized Holding Gains and Losses	The following tables summarize the estimated value of the Company’s cash equivalents and debt securities and the gross unrealized holding gains and losses (in thousands): September 30, 2018 Amortized cost Unrealized gain Unrealized loss Estimated Fair Value Cash equivalents: Money market funds $ 6,681 $ — $ — $ 6,681 Corporate debt securities — — — — Commercial paper 2,246 — — 2,246 Total cash equivalents $ 8,927 $ — $ — $ 8,927 Debt securities: Corporate debt securities $ 21,726 $ 1 $ (5 ) $ 21,722 Commercial paper 26,293 1 (3 ) 26,291 Total debt securities $ 48,019 $ 2 $ (8 ) $ 48,013 December 31, 2017 Amortized cost Unrealized gain Unrealized loss Estimated Fair Value Cash equivalents: Money market funds $ 19,612 $ — $ — $ 19,612 Total cash equivalents $ 19,612 $ — $ — $ 19,612 Debt securities: Corporate debt securities $ 6,503 $ — $ (2 ) $ 6,501 Commercial paper 3,244 — (1 ) 3,243 Total debt securities $ 9,747 $ — $ (3 ) $ 9,744

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Schedule of Components of Accrued Liabilities	Accrued liabilities consist of the following (in thousands): September 30, December 31, 2018 2017 Compensation and related benefits $ 1,049 $ 1,262 Contract research costs 196 634 Consulting costs 184 87 Franchise tax 30 56 Contract manufacturing costs — 4 Other — 41 Total accrued liabilities $ 1,459 $ 2,084

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Long-Term Debt and Unamortized Discount Balances	Long-term debt and unamortized discount balances are as follows (in thousands): September 30, December 31, 2018 2017 Face value of long term debt $ 25,000 $ 15,000 Exit fee 1,960 1,125 Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees (1,539 ) (1,032 ) Total long term debt 25,421 15,093 Less: current portion of long term debt (6,565 ) (2,002 ) Long term debt, net $ 18,856 $ 13,091

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Stock Option Activity	The following table summarizes stock option activity under the Company’s stock-based compensation plan during the nine months ended September 30, 2018 (in thousands, except option and share data): Shares Available for Grant Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (in Years) Aggregate Intrinsic Value Outstanding as of December 31, 2017 799,375 1,467,051 $ 12.70 8.41 $ 4,511 Additional options authorized 526,330 Granted (867,500 ) 867,500 $ 10.34 Exercised (39,780 ) $ 7.45 Canceled and forfeited 215,274 (215,274 ) $ 13.36 Outstanding as of September 30, 2018 673,479 2,079,497 $ 11.75 8.33 $ 4,243 Vested and exercisable as of September 30, 2018 842,968 $ 12.62 7.66 $ 2,080
Summary of Non-cash Stock Based Compensation Expense	Total stock-based compensation expense recognized for options granted was as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 344 $ 273 $ 1,138 $ 771 General and administrative 828 680 2,358 2,452 Total $ 1,172 $ 953 $ 3,496 $ 3,223
Employees Stock Option [Member]
Fair Value of Stock Option Granted Using Black-Scholes Option Pricing Model	The fair value of employee stock options was estimated using the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) — 5.77-6.08 5.27-6.08 5.27-6.08 Volatility — 79.0% 84.0%-84.5% 79.0%-80.0% Risk free interest rate — 1.93%-2.01% 2.35%-2.68% 1.63%-2.23% Dividend yield — — — —

Description of Business - Addit

Description of Business - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Oct. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2018USD ($)Segment	Dec. 31, 2017USD ($)
Description Of Business [Line Items]
Number of operating segments | Segment		1
Cash and cash equivalents		$ 16,927	$ 32,035
Debt securities, available-for-sale		48,013	9,744
Accumulated deficit		(154,696)	$ (118,806)
Net proceeds from issuance of common stock		$ 42,890
Underwriting Agreement [Member] | Subsequent Event [Member]
Description Of Business [Line Items]
Net proceeds from issuance of common stock	$ 48,000
Underwriting Agreement [Member] | Subsequent Event [Member] | Common Stock [Member]
Description Of Business [Line Items]
Number of shares issued under public offering | shares	4,830,918
Number of additional common stock issued | shares	630,120
Common stock, offering price per share | $ / shares	$ 10.35

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Earnings per Share (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	2,089,677	1,540,574	2,089,677	1,540,574
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	10,180	10,180	10,180	10,180
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	2,079,497	1,530,394	2,079,497	1,530,394

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets	$ 56,940,000	$ 56,940,000	$ 29,356,000
Financial liabilities	0	0	0
Transfers between levels		0	$ 0
Other-than-temporary impairment losses	0	0
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available- for-sale of debt securities contractual maturity			1 year
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets	0	0	$ 0
Financial liabilities	$ 0	$ 0	$ 0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Assets, Fair Value	$ 56,940,000	$ 29,356,000
Corporate Debt Securities [Member]
Financial assets:
Assets, Fair Value	21,722,000	6,501,000
Money Market Funds [Member]
Financial assets:
Assets, Fair Value	6,681,000	19,612,000
Commercial Paper [Member]
Financial assets:
Assets, Fair Value	28,537,000	3,243,000
Level 1 [Member]
Financial assets:
Assets, Fair Value	6,681,000	19,612,000
Level 1 [Member] | Money Market Funds [Member]
Financial assets:
Assets, Fair Value	6,681,000	19,612,000
Level 2 [Member]
Financial assets:
Assets, Fair Value	50,259,000	9,744,000
Level 2 [Member] | Corporate Debt Securities [Member]
Financial assets:
Assets, Fair Value	21,722,000	6,501,000
Level 2 [Member] | Commercial Paper [Member]
Financial assets:
Assets, Fair Value	28,537,000	3,243,000
Level 3 [Member]
Financial assets:
Assets, Fair Value	$ 0	$ 0

Fair Value Measurements - Estim

Fair Value Measurements - Estimated Value of Cash Equivalents and Debt Securities and Gross Unrealized Holding Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash Equivalents and Investment Securities [Line Items]
Cash equivalents, Amortized cost	$ 8,927	$ 19,612
Cash equivalents, Estimated Fair Value	8,927	19,612
Debt securities, Amortized cost	48,019	9,747
Debt securities, Unrealized gain	2
Debt securities, Unrealized loss	(8)	(3)
Debt securities, Estimated Fair Value	48,013	9,744
Money Market Funds [Member]
Cash Equivalents and Investment Securities [Line Items]
Cash equivalents, Amortized cost	6,681	19,612
Cash equivalents, Estimated Fair Value	6,681	19,612
Commercial Paper [Member]
Cash Equivalents and Investment Securities [Line Items]
Cash equivalents, Amortized cost	2,246
Cash equivalents, Estimated Fair Value	2,246
Debt securities, Amortized cost	26,293	3,244
Debt securities, Unrealized gain	1
Debt securities, Unrealized loss	(3)	(1)
Debt securities, Estimated Fair Value	26,291	3,243
Corporate Debt Securities [Member]
Cash Equivalents and Investment Securities [Line Items]
Debt securities, Amortized cost	21,726	6,503
Debt securities, Unrealized gain	1
Debt securities, Unrealized loss	(5)	(2)
Debt securities, Estimated Fair Value	$ 21,722	$ 6,501

Accrued Liabilities - Schedule

Accrued Liabilities - Schedule of Components of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Compensation and related benefits	$ 1,049	$ 1,262
Contract research costs	196	634
Consulting costs	184	87
Franchise tax	30	56
Contract manufacturing costs		4
Other		41
Total accrued liabilities	$ 1,459	$ 2,084

License, Collaboration and Pr_2

License, Collaboration and Product Development Agreements - Additional Information (Detail) - USD ($) $ in Thousands	Aug. 11, 2018	May 15, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development			$ 13,196	$ 6,145	$ 25,080	$ 21,740
Product Development Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Product development fees	$ 10,000
Expert consultant fees and pass through costs	800
Incentive-based regulatory milestone fees	$ 1,000
Number of common stock issued subject to quarterly vesting requirements	115,526
Product agreement termination notice period	90 days
Research and development					$ 400
Progeria Research Foundation (PRF) Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development		$ 2,000
Collaboration agreement initial term		10 years
Collaboration agreement subsequent renewal terms		2 years

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	May 31, 2018	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Aug. 03, 2018
Debt Instrument [Line Items]
Interest expense			$ 700,000	$ 1,600,000
Oxford Loan [Member]
Debt Instrument [Line Items]
Loan agreement, aggregate borrowing capacity		$ 25,000,000
Loan, maturity period				Jul. 1, 2021
Loan agreement, floating rate terms				Floating rate per annum equal to the greater of either the 30-day U.S. Dollar LIBOR reported in the Wall Street Journal plus 6.41% or 6.95%
Loan origination fees and debt issuance costs		400,000
Loan agreement, covenant noncompliance penalty				5.00%	5.00%
Loan agreement, covenant description				The Oxford Loan includes customary events of default, including failure to pay amounts due, breaches of covenants and warranties, material adverse effect events, certain cross defaults and judgments, and insolvency. If the Company is unable to comply with these covenants or if the Company defaults on any portion of the outstanding borrowings, the lenders can also impose a 5.0% penalty and restrict access to additional borrowings under the loan and security agreement.
Loan agreement, covenant compliance				The Company was in compliance with the terms under the Oxford Loan as of September 30, 2018 and December 31, 2017.
Prepayment of loan fee, description				Calculated as of the loan origination date, equal to (i) 3.0% of the loan prepaid during the first 12 months, (ii) 2.0% of the loan prepaid in months 13-24 and (iii) 1.0% of the loan prepaid thereafter.
Oxford Loan [Member] | Loan Prepaid During First 12 Months [Member]
Debt Instrument [Line Items]
Debt instrument prepayment fee percentage				3.00%
Oxford Loan [Member] | Loan Prepaid During 13 to 24 Months [Member]
Debt Instrument [Line Items]
Debt instrument prepayment fee percentage				2.00%
Oxford Loan [Member] | Loan Prepaid After 24 Months [Member]
Debt Instrument [Line Items]
Debt instrument prepayment fee percentage				1.00%
Oxford Loan [Member] | Minimum [Member]
Debt Instrument [Line Items]
Loan agreement, interest rate			6.95%	6.95%
Oxford Loan [Member] | Food And Drug Administration [Member]
Debt Instrument [Line Items]
Percentage of success fee				5.00%
Success fees payable maximum period after approval				30 days
Oxford Loan [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Loan agreement, percentage to be added to the interest rate under condition				6.41%
Oxford Loan [Member] | Tranche A [Member]
Debt Instrument [Line Items]
Face value of term loan		$ 15,000,000			$ 15,000,000
Interest only payment period		18 months
Principal and interest payment period		36 months
Percentage of exit fee on principal balance			7.50%	7.50%
Loan final repayment exit fees payable			$ 1,100,000	$ 1,100,000	$ 1,125,000
Oxford Loan [Member] | Tranche A [Member] | Food And Drug Administration [Member]
Debt Instrument [Line Items]
Fee payable maximum period from funding				10 years
Oxford Loan [Member] | Amended Tranche B [Member]
Debt Instrument [Line Items]
Principal and interest payment period	30 months
Extended term of interest only payment period	6 months
Percentage of exit fee on principal balance			7.50%	7.50%
Loan final repayment exit fees payable			$ 400,000	$ 400,000
Loan final repayment fee payable			100,000	100,000
Oxford Loan [Member] | Amended Tranche C [Member]
Debt Instrument [Line Items]
Loan final repayment exit fees payable			$ 400,000	$ 400,000
Amendment [Member] | Amended Tranche B [Member]
Debt Instrument [Line Items]
Face value of term loan	$ 5,000,000
Amendment [Member] | Amended Tranche C [Member]
Debt Instrument [Line Items]
Face value of term loan						$ 5,000,000

Debt - Schedule of Long-Term De

Debt - Schedule of Long-Term Debt and Unamortized Discount Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Less: current portion of long term debt	$ (6,565)	$ (2,002)
Long term debt, net	18,856	13,091
Oxford Loan [Member] | Tranche A, B and C [Member]
Debt Instrument [Line Items]
Face value of long term debt	25,000
Exit fee	1,960
Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees	(1,539)
Total long term debt	25,421
Less: current portion of long term debt	(6,565)
Long term debt, net	18,856
Oxford Loan [Member] | Tranche A [Member]
Debt Instrument [Line Items]
Face value of long term debt		15,000	$ 15,000
Exit fee	$ 1,100	1,125
Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees		(1,032)
Total long term debt		15,093
Less: current portion of long term debt		(2,002)
Long term debt, net		$ 13,091

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares
Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Shares Available for Grant, Outstanding, Beginning balance	799,375
Shares Available for Grant, Additional options authorized	526,330
Shares Available for Grant, Granted	(867,500)
Shares Available for Grant, Canceled and forfeited	215,274
Shares Available for Grant, Outstanding, Ending balance	673,479	799,375
Number of Options, Outstanding, Beginning balance	1,467,051
Number of Options, Granted	867,500
Number of Options, Exercised	(39,780)
Number of Options, Canceled and forfeited	(215,274)
Number of Options, Outstanding, Ending balance	2,079,497	1,467,051
Number of Options, Vested and exercisable	842,968
Weighted-Average Exercise Price, Outstanding, Beginning balance | $ / shares	$ 12.70
Weighted-Average Exercise Price, Granted | $ / shares	10.34
Weighted-Average Exercise Price, Exercised | $ / shares	7.45
Weighted-Average Exercise Price, Canceled and forfeited | $ / shares	13.36
Weighted-Average Exercise Price, Outstanding, Ending balance | $ / shares	11.75	$ 12.70
Weighted-Average Exercise Price, Vested and exercisable | $ / shares	$ 12.62
Weighted Average Remaining Contractual Life, Ending balance	8 years 3 months 29 days	8 years 4 months 28 days
Weighted-Average Remaining Contractual Life, Vested and exercisable	7 years 7 months 28 days
Aggregate Intrinsic Value | $	$ 4,243	$ 4,511
Aggregate Intrinsic Value, Vested and exercisable | $	$ 2,080

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted			867,500
Total unrecognized compensation expense	$ 9.7		$ 9.7
Weighted average period expected to recognized compensation expense (in years)			2 years 9 months 18 days
Employees Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted	0	56,000	799,500	579,700
Weighted-average grant date fair value of employee option grants		$ 7.45	$ 7.50	$ 7.66

Stock-Based Compensation - Fair

Stock-Based Compensation - Fair Value of Stock Option Granted Using Black-Scholes Option Pricing Model (Detail) - Employees Stock Option [Member]	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum		84.00%	79.00%
Volatility	79.00%
Volatility, maximum		84.50%	80.00%
Risk free interest rate, minimum	1.93%	2.35%	1.63%
Risk free interest rate, maximum	2.01%	2.68%	2.23%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 9 months 7 days	5 years 3 months 7 days	5 years 3 months 7 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Non-cash Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 1,172	$ 953	$ 3,496	$ 3,223
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	344	273	1,138	771
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 828	$ 680	$ 2,358	$ 2,452

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Tax expense	$ 0	$ 0	$ 0	$ 0

Legal Matters - Additional Info

Legal Matters - Additional Information (Detail) - 3 Putative Securities Class Action Complaints [Member] - Claim	Dec. 09, 2015	Jul. 31, 2015
Loss Contingencies [Line Items]
Loss contingency, new claims filed, number		3
Loss contingency, claims consolidated, number	3

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2017USD ($)ft²	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)
Other Commitments [Line Items]
Changes in commitments and contingencies		$ 0	$ 0
Palo Alto, California [Member]
Other Commitments [Line Items]
Total leased space | ft²	8,029
Lease commencement date	Mar. 1, 2018
Long-term operating lease expiry period month and year	2023-02
Lease renewal term	3 years
Security deposit of collateral for lease	$ 300,000
Operating leases, future minimum rent payable per year	$ 600,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Oct. 31, 2018	Sep. 30, 2018
Subsequent Event [Line Items]
Net proceeds from issuance of common stock		$ 42,890
Underwriting Agreement [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Net proceeds from issuance of common stock	$ 48,000
Underwriting Agreement [Member] | Subsequent Event [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Number of shares issued under public offering	4,830,918
Number of additional common stock issued	630,120
Common stock, offering price per share	$ 10.35